1. Field of the Invention
The present invention relates to a designing method of a zoom optical system, and more specifically, to a designing method of a zoom optical system including a gradient index optical element and or a diffractive optical element.
2. Description of the Related Art
Conventionally, a multitude of proposals have been made to reduce the size of a zoom optical system by decreasing the number of optical elements such as lens elements. In recent years, since the manufacture of an optical surface having an aspherical configuration has become comparatively easy, it has been enabled to reduce the number of optical elements constituting a zoom optical system by using the aberration correcting capability of an aspherical surface (U.S. Pat. No. 5,327,290).
However, aberration correction through the use of an aspherical surface is not effective for correction of all the aberrations. For example, aspherical surfaces have no aberration correcting capability for correction of Petzval sum and chromatic aberration associated with curvature of field. For this reason, even if a number of aspherical surfaces are used, it is theoretically impossible to form each zoom unit of only one optical element while maintaining the optical performance of the zoom lens system.
On the other hand, a zoom optical system has recently been proposed which performs aberration correction by using a gradient index optical element {so-called gradient index lens (GRIN lens) element} and a diffractive optical element (DOE).
However, although it is known that the aberration correcting capabilities of the gradient index optical element and the diffractive optical element are extremely high, handling of these elements in designing is not easy and it is extremely difficult to apply them to a zoom optical system in a manner such that their advantages are completely capitalized on.
Examples of conventional optical designing methods of an optical system including a gradient index optical element include a designing method disclosed in Optical Design Method Based on the Quasi-Equivalent Glass of the Radial GRIN Lens, Toshiyuki NAGAOKA, Hirofumi TSUCHIDA and Kimiaki YAMAMOTO, Optics, Vol. 21 (November, 1992), pp. 795-798. This method enables knowledge of design of a homogeneous lens to be applied to design of a GRIN lens as it is by regarding the GRIN lens as homogeneous glass having an effective refractive index and an Abbe number equivalent to those of the GRIN lens. However, the designing method described in this document only teaches to facilitate handling of the GRIN lens in designing and does not discuss a designing method of a zoom optical system which effectively uses the aberration correcting capability of the GRIN lens.
Examples of conventional optical systems comprising a small number of elements and including a diffractive optical element include an optical system disclosed in U.S. Pat. No. 5,629,799. This optical system uses the dispersive characteristic of the diffractive optical element for chromatic aberration correction and realizes an achromat with a single lens element. However, this patent does not refer to a zoom optical system to which it is more difficult to apply the diffractive optical element, and does not discuss a designing method of a zoom optical system which effectively uses the aberration correcting capability of the diffractive optical element.